Tens of thousands of objects are routinely tracked
through space. US and Russian communications satellites have collided in space
in the first such reported mishap.

Some
6,000 satellites have been sent into orbit since 1957.

A satellite owned by the US company Iridium hit a defunct
Russian satellite at high speed nearly 780km (485 miles) over Siberia on Tuesday,
Nasa said. The risk to the International Space Station and a shuttle launch
planned for later this month is said to be low. The impact produced a massive
cloud of debris, and the magnitude of the crash is not expected to be clear for
weeks.

The reportedly non-operational Russian satellite, weighing
950kg (2,094lb), had been launched in 1993, while the Iridium satellite weighed
560 kg and was launched in 1997.

When two such objects collide with such force, the ensuing
debris can destroy other satellites, says the BBC's Andy Gallacher in Florida.
But Nasa said the risk to the ISS and its three astronauts was low as the
station orbits the earth some 435km below the course of the collision.

It is hoped that most of the wreckage from the collision
will burn up in the earth's atmosphere, our correspondent says. Hundreds of
pieces of wreckage are now being tracked, reports say, adding to the tens of
thousands of objects that are routinely tracked through space.